Chromizing, adhering coating



May 27, 1958 G. A. SAMUEL CHROMIZING, ADHERING COATING Filed April 10. 1956 INVENTOR genesis CHRoMtziNo, Annahme coATlNG George A. Samuel, White Plains, N. Y., assgnor to Metal Diffusions, Inc., Philadelphia, Pa., a corporation of New Jersey Application April 10, 1955, Serial No. 577,297

1 Claim. (Cl. 117-403) The present invention relates to chromizing of ferrous metal articles, especially sheet, plate and strip steel.

The purpose of the invention is to simplify the chromizing of ferrous metal articles by avoiding the necessity of packing a chromizing compound in a box around the articles.

A further purpose is to permit chromizing ferrous metal articles in existing furnaces or in modified furnaces having atmosphere control.

A further purpose is to accelerate the chromizing of ferrous metal articles by obtaining intimate adherence between a coating of chromizing compound and the surface to be chromized, while at the same time permitting ready'removal of the residue which is rendered friable after chromizing.

A further purpose is to employ a stagnant or substantially quiescent relatively light protecting gas which will exert a minimum of washing action which might dissipate heavier chromizing gas.

A further purpose is to place the work in a container or containers open at the top which will trap chromizing gas and prevent its dissipation in the effluent protecting gas.

Further purposes appear in the specication and in the claim.VV

In thedrawings I have chosen toillustrate a few only' of the numerous embodiments in which my invention may appear, ,selecting the formsshown fromthe standpoints of convenience in illustration, satisfactory operation and clear demonstration of the principles involved. Figure l is a diagrammatic central vertical section of one form of furnace useful in the invention.

VFigure 2 is a similar view showing a modification.

Describing in illustrationrbut not in limitation and referring to the drawings:

Chromizing as usually carried out at the present time involves packing of a powdery chromizing composition around ferrous metal articles placed in a chromizing box. Chron'iizing by this technique 'is tedious and' expensive because of the labor involved in packing, and

because of the danger that in certain types of Work a high rejection rate may result through inability to obtain uniform and pore free chromizing; I

It is particularly difficult to chromize sheet and strip by packing ,becauseY of the labor involved in spreading the powder in a position to adjoin the side of the sheet.

I have discovered that a relatively simple procedure can be 4used to obtain uniform, inexpensive chromizing of many :ferrous metal articles including sheet, plate and strip. The invention does not require box techniques as at present practiced and lends itself toapplication in atmosphere heating furnaces, car-bottom furnaces, bell furnaces, muflie furnaces and the like, and on chain and roller hearth furnaces having atmosphere control.

Thus the cost of additional equipment can be reduced to a minimum, while the results obtained are comparable to the best pack methods.

riice The invention is applicable toa wide variety of plain carbon and alloy steels, whether in low, intermediate or high carbon ranges, although for most ductile coatings low carbon is preferred, of the order of 1010. The invention is also applicable to sheet, strip, plate, fabricated forms, Wire, rod, bar and tubing, as well as a wide variety of other forms of steel.

In accordance with the invention, an adhering chromizing coating is applied which .forms a deposit on the surface of the steel to be chromized. It will be evident, of course, that the coating can be applied by any suitable technique such as roller coating, doctoring, spraying, dipping, brushing, or the like. Where desired, the coating can be applied selectively and if other surfaces are not brought in contact with coating they will not be chromized. For example, one side only of a sheet can be coated and in making a stack the two coated sides of adjoining sheets will be placed together, placing two noncoated sides of adjoining sheets together. The sheets will be chromized then only on one side.

Considering rst the form of Figure 1, I make up an adhesive coating or slurry of chromizing compound which has the inherent property of becoming friable by the end of the chromizing operation. The slurry is non-carbonaceous, and therefore has the advantage that it does not carburize the work.

Since the coating is applied directly to the surface of the work, the chromizing gas, which will include lluorine, does not have a tendency to travel great distances in high concentrations, where pickling and etching eects might be obtained, and undergoes a minimum of reactions, but performs very effective transfer of chromium to the work, thus giving better penetration in a given time.

The result is that a minimum of undesired ferrous halide is produced.

The catalyst used is a compound of chromium and flourine. While any one of the uorides of chromium may be employed, it is decdedlypreferable for several reasons to use a complex chromium ammonium fluoride. Onea'dvantage is that this compound breaks down, liberating hydrogen and nitrogen and tending to sweep out air which might otherwise be entrapped adjacent the work. Another advantage is that when the compound breaks down, chromium or chromium and halide is available in nascent form.

While chromium jiuoride or a complex ammonium chromium liuoride can be used from any suitable source of such materials, it is preferable to produce the catalyst by a reaction in making the compound. This is desirably accomplished by reacting ammonium biuoride in water v to deal with explosive conditions, as it occurs violently peratures which mightotherwise occur; `The inert'body? ing material maybe finely divided alumina, silica or clay,

such as kaolin. used.

The slurry or dispersion has some remarkably advantageous properties. When coated by brushing, spraying, dipping or otherwise on the ferrous metal article and allowed to dry, the adhesion is so firm that the work can Any other inert bodying powder may be Yto its .original volume.

4 Y Percent Ammonium bifluoride j 1.4V Ferrochromium (preferably the 65 percent by weight chromiumV grade) 67.5 Inert bodying ingredient, in this case alumina powderV 17 Water Balance In making up this composition, it is preferable to proceed as follows: i Y Y Dissolve lY parts by weight of ammonium biuoride in 90 parts' by Weight of water. Make up a mixture of 80 percent by weight of Aferro chromium powder and 20 Vpercent by weight of alumina. 6 parts by weight of this mixture is, added by stirring to 1 part by weight of thesolution` of ammonium biiluoride.y The violent reaction with evolution ofV gas and Vheat continues in some cases for as much as two hours. Allow they resulting slurry to. cool. VIt is then readyV for use. If` preferred, it can be allowed to dry and then made up with'water The dry powder can ,be shipped as such and mixed with water at la distant point.

There is of course a great excess ofV chromium in the above mixture and the quantityof ammonium biuorideis notl critical. l Good results can be obtained using as little as 0.1 percent by weightofammonium biuoride or as much as Y percent by weight of ammonium 'bitluo` ride, ladjusting the quantity of water if desired to obtain a suitable slurry. g j

p The slurry should beviolently agitated to maintain preferably hydrogen or split ammonia, but permissibly nitrogen or other suitable inert protecting-gas, is used. The inert protecting gas should preferably be a gas whose density is relatively light comparedto that of iluorine. The inert protecting gas is'preferably introduced atfthe bottom by pipe 24 and taken out the top by'pipe 25;

It is very desirable that the gas streamv should be merely` great enough to make up for Yleakage so that a relatively quiescent or stagnant gas atmosphere remains around the work. This protects against removal of uon'ne or chromium fluoride vapor by washing or sweeping action which would seriously interfere with the chromizing;

In some cases it is preferable to further protect against dissipation of chromizing gas by placing theY work 20'l as shown in'Figure12 in a` container 26 which isv closed at the bottom and sides and open atf'the top,`and suitably has a coverV 27 which is not hermetically sealed but merely '4 loosely ts on the top. This further reduces the possibility of dissipatingthe chromizing gas by washing or sweeping action since the relatively heavychromizing gas tendsV to till the lower container portion 26,.

In Figure 1, the Work is shown as a stack of Ysheets while as in Figure 2 I illustrate the work consisting of a roll of strip held by a band 28. It will be evident thatV the lstrip has beensuitably coated with chromizing com'- poundwhile unwound and then after drying has been wound Vup in roll form.

The chromizing temperature may be between 167001 and 2200 degrees Fahrenheit, preferably between 1650 and l950degrees Fahrenheit, and most desirably about 1750 degrees Fahrenheit. The high temperatures are usually to be avoided because of grain growth. The Time of chromizing is suitably in excess of 3 hours, and usually in excess of 5 Vhours, in many cases being as long as about 18 hours.

The final chromizing coating is bright andrmly adherent, while the powder deposit on the .other hand scales off readily. Y Y

The improvement in penetration according tothe pres-- ent invention' is of the orderl of 50 to 75 percent over the pack method as normally applied. Y

' The deposit obtained can be of any desired thickness inthe range from" 0.0001'inch to 0.010 inch. The deposit will commonly be of the order of 0.001 inch.

The particles used in making up the slurry should preferably be throughV 100 mesh per linear inch.

In operation, it will be evident that it is merely neces-A sary to coat the work with the chromizing compound,Y allow thecompound to dry, then insert the work in the furnace, heat the work up to chromizingV temperature, cool the work down' to .a safe temperature, remove the work from the furnace and then clean off the residual powder. 'The factthat'two articles are touching inY a stack or coil is not objectionable, as Vthepowder protects against damage.

Athe ferro chromium powder Vand the alumina in'dispersion v i when coating thework.`

In view of my invention and `disclosure variations and modifications to meet individualwhim'or particular need will doubtless become evident to others skilled in` the art, to obtain all orfpart of the benefits .ofmy invention without copying the method shown, and I therefore Y claim all such insofar as they fall within the reasonable spirit and scope of myiinvention. n Having thus described my invention, "what new yandjdesire to secure by Letters Patent is:

The method of chromizingferrous metal articles, which comprises coating therarticles with a watery dis- I claim as persion of chromium, the complex fluoride of ammonium Y and'chro'mium and an inert bodying agent, to for-m an Y adhesive bonded chromizing layer on the ferrous metal articles, and heating the ferrousmetal articles to a temperature of 1600 tof2200d'egrees F. andmaintaining the articlesyin a ,closed space at that temperaturevr Ywhile vexposed to and protected by the gas atmosphere Vevolved from the complex uoride to diifuse chromiuminto theV articles and render the coating friable so that it can be easily removed.

References Iitetl in the( fileof this Vpatent UNITED STATES PATENTSy 1,746,924 VBendix Feb.1;1`1, 1930 u oTHER REFERENCES Y Protective Coatingsfor Metals, by Burns and Brad-V ley, published by Reinhold (New York), 1955 (page 230A relied on).A 

